Ionic Bonding and Ionic Compounds Chemistry – Chapter 6.3 Ionic Bonding and Ionic Compounds

Objectives Compare and contrast a chemical formula for a molecular compound with one for an ionic compound. Discuss the arrangements of ions in crystals. Define lattice energy and explain its significance. List and compare the distinctive properties of ionic and molecular compounds. Write the Lewis structure for a polyatomic ion given the identity of the atoms combined and other appropriate information.

Ionic Compounds Ionic Compound: composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal. Most ionic compounds exist as crystalline solids. A crystal of any ionic compound is a three- dimensional network of positive ions (cations) mutually attracted to negative ions (anions).

Formula Unit The chemical formula of an ionic compound shows the ratio of the ions present ina sample of any size. Formula Unit: The simplest collection of atoms from which an ionic compound’s formula can be established. Ex. NaCl  sodium chloride. The ratio of ions in a formula unit depends on the charges of the ions combined.

Formation of Ionic Compounds Electron-dot notation can be used to demonstrate the changes that take place in ionic bonding. Nature favors arrangements in which potential energy is minimized. In an ionic crystal, ions minimize their potential energy by combining in an orderly arrangement known as a crystal lattice.

Lattice Energy To compare bond strengths in ionic compounds, chemists compare the amounts of energy released when separated ions in a gas come together to form a crystalline solid. Lattice Energy: The energy released when one mole of an ionic crystalline compound is formed from gaseous ions. Negative energy values indicate energy is released when the crystals are formed. Lattice Energies of Some Common Ionic Compounds Compound Energy (kJ/mol) NaCl -787.5 NaBr -751.4 LiF -1032 If the lattice energy in compound A is greater than that of compound B, it will be more difficult to break the bonds in compound A.

Forces of Attraction The force that holds ions together in ionic compounds is a very strong overall attraction between positive and negative charges. The forces of attraction between molecules in a molecular compound are weaker than the attractive forces in ionic bonding. The melting point, boiling point, and hardness of a compound depend on how strongly its basic units are attracted to each other.

Polyatomic Ions Certain atoms bond covalently with each other to form a group of atoms that has both molecular and ionic characteristics. Polyatomic Ions: A charged group of covalently bonded atoms. Polyatomic ions combine with ions of opposite charge to form ionic compounds. The charge of a polyatomic ion results from an excess of electrons or a shortage of electrons.